Silver halide photographic emulsions in their primitive state are typically too insensitive and low in contrast for use in practical application. Emulsions of fine grain size (under about 0.5.mu. mean grain diameter) in particular are relatively insensitive but offer advantages such as high covering power and resolution. Many methods of increasing the sensitivity of silver halide photographic emulsions have been advanced. These methods fall into two general classes:
1. Chemical sensitization in which the emulsion is digested with agents such as sulfur compounds, reducing agents, salts of noble metals, and combination of such agents, to bring about a chemical change in the surface of the silver halide grain; PA1 2. optical or spectral sensitization in which sensitizing dyes are incorporated into the emulsion, resulting in an increase in the range of spectral sensitivity of the system. PA1 J. thiele and G. Steimmig, Ber. Deut. Chem., 40, 955 (1907); PA1 G. schwarzenbach and K. Lutz, Helv. Chim. Acta, 23, 1139. 1147 (1940); PA1 J. a. barltrop, C. G. Richards, D. M. Russell and G. Ryback, J. Chem. Soc., 1132 (1959); PA1 J. o. halford and R. M. Fitsch, J. Amer. Chem. Soc., 85, 3354 (1963); PA1 D. lloyd, R. H. McDougall and D. R. Marshall, J. Chem. Soc., 3785 (1965).
The first method, chemical sensitization, usually provides only moderate speed increases before reaching a point at which further increases in sensitizer result in undesirable side effects such as poor keeping quality and fog. The second method, spectral sensitization, provides greater speed increases by extending the spectral range of sensitivity, but it is only effective when the energy of the exposing source includes wavelengths absorbed by the sensitizer. In addition, spectral sensitization may result in safelight fog unless the materials are handled only in total darkness.